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Střihové napětí na stěně pod velkou bublinou stoupající v nakloněném plochém kanále
Tihon, Jaroslav ; Pěnkavová, Věra ; Vejražka, Jiří
The motion of single air bubbles in an inclined flat water channel is investigated experimentally. The electrodiffusion technique of near-wall flow diagnostics is applied to measure the wall shear stress distribution under large bubbles rising along the upper channel wall. The electrodiffusion measurements are synchronized with the visual observation of bubble movement done by a high-speed camera. The analysis of video records provides information on the bubble shape and terminal velocity. The experiments are carried out for three different channel heights and cover a wide range of channel inclination angles (from horizontal to vertical position). The experimental results on bubble motion characteristics obtained in stagnant and upward-flowing liquid arrangements are confronted with the recent predictions and scaling provided by theoretical models.
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Vliv viskoelastických vlastností mezifázového rozhraní na oscilace bubliny
Vejražka, Jiří ; Vobecká, Lucie ; Zedníková, Mária ; Orvalho, Sandra ; Tihon, Jaroslav
Shape oscillations of bubbles in surfactant solutions are investigated by means of high-speed imaging. Two cases are studied: bubbles attached to a capillary tip, and bubbles freely rising after detachment. The frequency f and the decay time τ of shape oscillations are evaluated, and their dependence on the surfactant concentration and on the bubble size is investigated. If the surfactant is present, τ shortens and a minimum is found for a certain concentration. This observation demonstrates that in the surfactant solutions, the energy dissipation linked to interfacial effects can be much more important than the dissipation due to liquid viscosity. The frequency f increases with increasing concentration of the surfactant first, and then it drops rapidly at concentration, at which τ is minimal. The initial increase of the oscillation frequency suggests that the interfacial elasticity can play an important role in surfactant solutions.
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Vliv pulzací nátoku na strukturu toku v kanále za náhlým rozšířením.
Pěnkavová, Věra ; Tihon, Jaroslav
The wall shear rate profiles were measured along the bottom and roof walls of rectangular channel with backward-facing step. The profiles were measured by two-strip electrodiffusion probes. It is possible to see the strong effect of the inlet flow pulsations on the flow structure. The influence of the inlet flow pulsation was studied for various frequence and amplitude for water when the Reynolds number was ranging from 100 to 1500 and expansion ratio of the channel was 2.
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Numerické modelování impaktního proudění tekutiny na stěnu a experimentální ověření výsledků pomocí teplotní anemomerie
Cvetinović, D. ; Tihon, Jaroslav ; Vejražka, Jiří ; Drahoš, Jiří
The configuration of a submerged, circular, unconfined impinging air jet, which issues from a bell-shaped converging nozzle, is characterized numerically and experimentally. A commercial CFD package is repeatedly employed to simulate three different configurations to validate the different turbulence models, namely k–? Re-Normalized Group (RNG), k–? Shear Stress Transport (SST) and the Reynolds Stress Model (RSM). A hot-wire anemometry technique is applied to investigate extensively the flow field of the impinging jet. The comparison of results suggests that the k–? SST turbulence model is the most reliable for the flow configurations under investigation.
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Diagnostika toku v kanále za náhlým rozšířením
Tihon, Jaroslav
A backward-facing step flow is studied by measuring the instantaneous wall shear rate along the channel certenline. A two-step electrodiffusion probe is applied to provide information on the flow direction in the near-wall region. The wall shear rate profiles behind the step are measured and the position of reattachment point detected. The Reynolds numbers examined cover the laminar, transient and beginning of turbulent flow regime. The results are found to be in a good agreement with the experimental and numerical data previously published for the same channel geometry.
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